I'm not sure your logic follows in this case: you seem to be saying, "because in contemporary English the dummy pronoun cannot be dropped, that automatically means that in the period when the 3rd person ending was -(e)th, it also couldn't be dropped".
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Neil CoffeyAug 24 '11 at 12:49

“Sufficeth” is just an old spelling of “suffices” commonly used in the King James translation of the Bible and other Renaissance religious texts. People often use it in a joking manner to give their writing a semi-Biblical air, especially in the phrase “it sufficeth to say.” But they sound clumsy rather than clever when they omit the “it” and begin the phrase thus “Sufficeth to say....”

@A.Uysal, the King James Bible was first published in 1611, and language has shifted somewhat since then. The -eth endings gradually shifted to an -s ending (e.g. "goeth" to "goes", "thinketh" to "thinks".) While you are correct about the meaning, I wanted to point out that the usage was common to most (if not all) texts of the era including Shakespeare, and not confined to only religious texts.
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Andrew NeelyAug 24 '11 at 11:49